The present invention relates to an ink jet printing apparatus capable of controlling the viscosity of ink which is circulated therein and, more particularly, to an ink jet printer of the type which constantly or periodically discards a predetermined amount of ink and supplies the same amount of fresh ink to maintain the total ink viscosity within an allowable range.
In an ordinary ink jet printing apparatus, part of ink droplets ejected from an ink ejection head which does not contribute to data printout is caught by a gutter and collected thereby into an ink supply source for reuse. A problem has existed in such an apparatus in that the collected ink, which is the composition of droplets flown at a high speed through the space, has lost part of its volatile component during the flight of the droplets and thereby increases the ink density as a whole. In this manner, the content of the volatile component (solvent) in the ink decreases every time the ink is ejected from the nozzle as a result that the proportion of the nonvolatile component to the whole ink to be reused will progressively increase in the course of a long time of operation. The result will be an increase in the viscosity of the ink which prevents droplets to be formed adequately and eventually renders the apparatus inoperable.
To overcome this problem, there has been proposed a device which senses an ink voscosity and, upon increase thereof beyond a reference value, adds a diluted solution to the ink. Although successful to maintain a constant ink viscosity, such a prior art device makes the entire printer construction complicate due to the additional means for sensing an ink viscosity and means for supplying a diluted solution.